In recent years, a genetic test is becoming rapidly widespread in the field of clinical diagnosis. As one example of the genetic test, there is the diagnosis of lymph node metastasis of cancer. Cancer cells leave from a primary focus and spread via blood vessels and lymphatic vessels to the whole body. In operation for cancer, the focus should be removed as accurately as possible, and thus the metastasis should be accurately detected, and the cancer should be suitably treated depending on the degree of metastasis. Accordingly, the diagnosis of lymph node metastasis of cancer cells during the operation is of very significant significance. As one of the techniques of diagnosing the lymph node metastasis of cancer, there is a method of detecting, as a target nucleic acid, a nucleic acid for a protein unexpressed or expressed in a low amount in normal cells, but expressed in a large amount in cancer cells. By development of gene analysis techniques in recent years, a target nucleic acid contained in lymph node tissue excised from the living body can be amplified and detected thereby effectively diagnosing cancer.
For examining the metastasis of cancer to specific tissue by the genetic test, studies on genetic test of cancer by an LAMP method (loop-mediated isothermal amplification method) or PCR (polymerase chain reaction) have been extensively conducted. This genetic test can be carried out by detecting a cancer marker contained in a tissue or a cell (for example, mRNA for a protein expressed specifically in a cancer cell).
A cancer marker (also referred hereinafter to simply as “marker”), cytokeratin 19 (CK19), is known to be useful as a marker for judging the lymph node metastasis of breast cancer. CK19 is a molecule recognized to show a significant difference in the expression level between a normal lymph node and a breast cancer cell that has metastasized to a lymph node.
In Asian countries such as China, Japan and Korea and in South America, there are many patients with stomach cancer. According to the statistics of cancer mortality in 2003 in Japan, stomach cancer is ranked second in males following lung cancer and second in females following colon cancer. Stomach cancer is a furtive disease, thus making early detection difficult, and develops often to metastasize to lymph nodes etc.
Conventionally, cytokeratin 20 and carcinoembryonic antigen have been used as markers for judging the lymph mode metastasis of stomach cancer. However, there are few reports on the genetic test of stomach cancer, and there has been demand for further development of a method for judging the lymph node metastasis of stomach cancer.